Patients with psychiatric disorders such as schizophrenia commonly present with impaired language. Here we investigate language in schizophrenia with a focus on inflectional morphology, using an intensively studied and relatively well-understood linguistic paradigm. Patients with schizophrenia (n = 43) and age-matched healthy control subjects (n = 42) were asked to produce past tenses of regular ("slip"), irregular ("swim"), and novel ("plag") English verbs. Patients were impaired at regulars and novels ("slipped, plagged"), with relative sparing of irregulars ("swam"), controlling for numerous subject- and item-specific factors (e.g., IQ, phonological complexity). Additionally, patients' thought-disorder scores significantly predicted their performance at regular and novel (but not irregular) past-tense production. The results support grammatical deficits in schizophrenia, with a relative sparing of lexical memory, and suggest that thought disorder may be linked with grammatical impairments in the disorder. (Contains 4 tables.)